2.1.2 Thinking Ahead

Question 1

What is thinking ahead ?

Answer 1

• identifying preconditions of a system ; inputs outputs and reusable components
• by thinking ahead, developers can build programs that are easy and intuitive to use

Question 2

Inputs - all computational problems consist of inputs which are processed to produce an output

Answer 2

• include any data required to solve the problem
• these are entered into the system by the user
  consider data structure and data types involved

Question 3

Outputs

Answer 3

• are the results that are passed back
• outputs are produced once inputs have been processed / are essentially the solution to the problem

Question 4

Example of outputs

Answer 4

• visual onscreen information/ printed data
• audio
• haptic feedback
• data to be input into another process or system

Question 5

In which order is inputs and outputs determined?

Answer 5

• designers begin by considering outputs based on the users requirements
• the outputs are then used to identify the inputs required + how they need to be processed to produce the output

Question 6

Preconditions

Answer 6

• requirements which must be met before the program can be executed
• preconditions can be tested for within the code or included In the documentation for a particular library, subroutine or program

Question 7

Preconditions benefits

Answer 7

• including preconditions within documentation reduces the length and complexity of the program and saves time spent on debugging and maintenance
• preconditions make subroutines more reusable

Question 8

predictions that can be specified in the documentation can include :

Answer 8

• requirements for the device specification to be able to run the program
• the operating system required to run the program
• any necessary additional software is hardware

Question 9

Preconditions in subroutines/ programs

Answer 9

• the parameter list must not be empty, otherwise index out of bounds error will occur
• the data may need to be all of the same data type - otherwise invalid results may be returned or invalid comparisons made
• actions need to be validated to ensure that they will not crash the program if erroneous results are returned

Question 10

Reusable components

Answer 10

• commonly used functions which are packaged into libraries for reuse [1]
• Reusable components include :
• classes/objects [1]
• subroutines eg. Functions and procedures [1]
• abstract data structures e.g stacks and queues

Question 11

How are reusable components used when developing large programs ?

Answer 11

• programmers make use of reusable components when developing large programs by :
• designing software that is modular by breaking the problem into smaller manageable modules [1]
• examples of modular components include objects and functions [1]
• modules/modular components can be imported and reused in new software projects [1]
• or modular components can be shared at runtime through program libraries [1]

Question 12

Reusable components benefits

Answer 12

• modules are reliable and bug free as they have already been thoroughly tested [1]
• thus reusing tested modules leads to more reliable programs [1]
• utilising reusable modules reduces development time [1]
• as the programs can be shorter since the modules do not have to be rewritten + have already been tested [1]
• and modules can also be shared [1]

Question 13

Caching

Answer 13

• storing frequently used data/instructions in cache in case it is needed again [1]
• allowing faster access for future use as it saves time of retrieving instructions from secondary storage again [1]
• e.g frequently accessed webpages are cached so content can be loaded quicker

Question 14

Caching benefits

Answer 14

• frequently accessed webpages are cached so content can be loaded quickly and this frees up bandwidth for other tasks on a network e.g any images and text do not need to be downloaded every time the page is visited
• caching provides faster data retrieval by storing freq accessed data in the cache for quick access
• reduces load on servers and frees up network bandwidth by minimising repetitive data fetching

Question 15

Prefetching

Answer 15

• when caching algorithms predict which instructions are likely to be soon fetched or required by a program and these instructions/data are loaded and stored in cache
• this minimises delays waiting for data or instructions to be fetched from storage
• however prefetching is limited by the accuracy of the algorithms used as data stored in cache is not always used

Question 16

The size of the cache determines how effective caching is

Answer 16

• smaller caches limit how much data can be stored
• larger cache takes longer to search